This invention relates to a connector, a hook and a device for holding and connecting an anode rod in contact with an anode beam of an aluminium electrolytic cell, and a method for holding and connecting such an anode rod.
Aluminium metal is produced industrially by electrolysis of alumina in solution in an electrolyte bath using the Hall-Héroult process. The electrolyte bath is contained in an electrolysis pot comprising a steel shell that is coated on the inside with refractory and/or insulating materials, and at the bottom of which a cathode assembly is located. Anodes, typically made of a carbonaceous material, are partially immersed in the electrolyte bath.
Each anode is provided with a metallic rod that will be used for electrically and mechanically connecting it to an anode beam free to move with respect to a portal frame fixed above the electrolysis tank. Each anode rod is connected to the anode beam using hooks on each side of the anode rod and to a removable connector that can be placed on these hooks and that can bring the anode rod into contact with the anode beam.
Anode rods are connected to the cathode frame in different cases during the life of the installation:                during start up of the installation, just at the beginning of steady state operation, after the preheating phase;        during steady state operation, when the anode beam has reached the lower limit of its travel distance and it needs to be raised above the set of anodes in order to continue operation;        when the anodes are too spent and/or deteriorated and they have to be replaced.        
It can sometimes happen that the anode rods are badly positioned during this connection operation, for example they might be slightly oblique instead of being perfectly vertical. This problem arises particularly when the anodes are replaced, since in this case they can be held in this incorrect position due to the crust formed on the surface of the electrolyte bath. It is difficult to detect this bad positioning due to the apparent support provided by the crust, and the connector appears to be fulfilling its purpose perfectly well.
However, under steady state operation conditions, the crust melts and then can no longer hold the anode rod in position. The anode rod is then no longer satisfactorily held in contact with the anode beam by the connector due to the incorrect initial positioning. The result is deterioration of the electrical and mechanical connection, and this deteriorates the performances of the installation. In the worst case, the anode rod can slide with respect to the anode beam and fall into the bottom of the electrolytic pot, thus requiring long and expensive operations.
In general, this problem arises when the anode position changes for different reasons. The forces applied to the anode rod are then modified from the forces in the initial position in which the clamping of the anode rod was satisfactory. Consequently, the connector no longer satisfactorily holds the anode rod in contact with the anode beam.
Documents EP 0 178 766 and U.S. Pat. No. 5,876,585 describe temporary connection devices that clamp the anode rod in contact with the anode beam when the installation is being started up. These connection devices comprise means such as rollers and spring or Belleville washer type systems enabling relative displacement of the anode rod while maintaining satisfactory clamping at the different expansion and deformation stages of the elements in the electrolytic cell being preheated.
However, these devices are only designed for use in the preheating phase of the installation during which the anodes are put down on the bottom of the pot. During continuous operating conditions, when the anodes are no longer supported on the bottom of the pot, the rollers would not be capable of providing sufficient clamping of the anode rods. Thus, during steady state operation conditions, document U.S. Pat. No. 5,876,585 describes the use of connection devices without any compensation means such as rollers and springs.
Furthermore, these devices are not sufficient to compensate for bad initial positioning of the anode rod with respect to the anode beam, because in this case, the consequences on the displacement of the rod are much greater than the consequences of an increase in temperature of the components of the cell.
French application FR 2 039 543 describes a connector with a body comprising two levers that will apply pressure on the anode rod towards the anode beam through at least one bearing face, two substantially coaxial lateral rods each projecting beyond the sides of the said body, and each designed to rest on a hook fixed to the anode beam on each side of the rod, and a clamping screw that can make the levers pivot about the axis of the lateral rods, such that the connector can be in two particular geometric configurations; a clamping configuration in which the bearing face of the levers is in contact with the anode rode and forces it towards the anode beam substantially perpendicular to the anode beam, and a releasing (or “release”) configuration in which the bearing face of the levers does not apply any pressure on the anode rod. This type of connector is currently used in electrolysis workshops both in preheating and under continuous operating conditions, but it does not have any compensation means of allowing significant relative movement of the anode rod while maintaining satisfactory clamping of the anode rod on the anode beam.